Structural stability of Na-inserted spinel-type sodium titanium oxide

Abstract

Spinel-type sodium titanium oxide (Na3LiTi5O12, NTO), which has an analogous structure to Li4Ti5O12 (LTO), is prepared as a single-phase material for the negative electrode of Na-ion batteries. A superior Na insertion and extraction cycle performance is achievable based on its similarity with the LTO reaction mechanism. However, the detailed structure of the NTO material in the Na insertion state remains obscure. Consequently, the crystallographic features of the Na insertion mechanism have not been sufficiently elucidated. In this study, the structural analyses of NTO and Na-inserted NTO (Na-NTO) electrodes were performed via X-ray diffraction and Rietveld refinement. The Na occupation site of the NTO spinel lattice was altered from the oxygen tetrahedral to the octahedral site by the electrochemical Na insertion reaction. The lattice constants of NTO and Na-NTO were refined for aNTO = 8.73 Å and aNa-NTO = 8.84 Å; approximately 1% lattice expansion, which was caused by the local compressions of the Ti–O atomic distance existing in the NTO and their relaxation in the Na-NTO, was confirmed by Na insertion. Notably, Ti–O atomic distances in NTO are quite heterogeneous with values of 1.8 Å and 2.2 Å, while they are homogenized to 2.0 Å, 2.1 Å, and 2.2 Å in the Na-NTO lattice. Furthermore, the oxygen position in the NTO lattice is modified to enhance lattice symmetry by Na insertion, similar to the Li insertion reaction of the LTO lattice. Thus, the Na-NTO lattice structure is considerably more stable than the NTO lattice, which promises stable Na insertion and extraction cycle performances for NTO materials toward Na-ion battery utilization.